SECOND GREAT DIVISION OF THE ANIMAL 

 KINGDOM. 



ANIMALIA MOLLUSCA*. 



THE Mollusca have neither an articulated skeleton nor a vertebral 

 canal. Their nervous system is not united in a spinal marrow, but 

 merely in a certain number of medullary masses dispersed in different 

 points of the body, the, chief of which, termed the brain, is situated 

 transversely on the resophagus, and envelopes it with a nervous collar. 

 Their organs of motion and of the sensations have not the same uni- 

 formity as to number and position, as in the Vertebrata, and the 

 irregularity is still more striking in the viscera, particularly as respects 

 the position of the heart and respiratory organs, and even as regards 

 the structure of the latter ; for some of them respire elastic air, and 

 others salt or fresh water. Their external organs, however, and those 

 of locomotion, are generally arranged symmetrically on the two sides 

 of an axis. 



The circulation of the Mollusca is always double; that is, their 

 pulmonary circulation describes a separate and distinct circle. 



The blood of the Mollusca is white or bluish, and it appears to 

 contain a smaller proportionate quantity of fibrine than that of the 

 Vertebrata. There are reasons for believing that their veins fulfil 

 the functions of absorbent vessels. 



Their muscles are attached to various points of their skin, forming 

 tissues there which are more or less complex and dense. Their 

 motions consist of various contractions which produce inflexions and 



* Soft animals. 



